1. Technical Field
The field of the invention is within motor vehicles, and more specifically the control over distribution of torque generation by a plurality of electric motors located throughout the vehicle structure.
2. Description of the Related Art
It is known that torque vectoring is provided to all wheel drive vehicles. Torque vectoring is a known technology employed in automobile differentials. A differential transfers engine torque to the wheels. Torque vectoring technology provides the differential with the ability to vary the power to each wheel. This method of power transfer has recently become popular in all-wheel drive vehicles. Some newer front-wheel drive vehicles also have a basic torque vectoring differential. As technology in the automotive industry improves, more vehicles are equipped with torque vectoring differentials.
Differentials are known to refer to a particular type of simple planetary gear train that has the property that the angular velocity of its carrier is the average of the angular velocities of its sun and annular gears. This is accomplished by packaging the gear train so it has a fixed carrier train ratio R=−1, which means the gears corresponding to the sun and annular gears are the same size. This can be done by engaging the planet gears of two identical and coaxial epicyclic gear trains to form a spur gear differential. Another approach is to use bevel gears for the sun and annular gears and a bevel gear as the planet, which is known as a bevel gear differential.
The fundamental concept of torque vectoring depends on the principles of a standard differential. A differential shares available torque between wheels. This torque sharing ability improves handling and traction. Torque vectoring differentials were originally used in racing. The technology has slowly developed and is now being implemented in a small variety of production vehicles. The most common use of torque vectoring in automobiles today is in all-wheel drive vehicles.
The main goal of torque vectoring is to vary a share of torque between or among wheels coupled to a motor or engine. Differentials generally consist of only mechanical components. A torque vectoring differential often includes an electronic monitoring system in addition to standard mechanical components. This electronic aspect is only to direct the mechanical differential when and how to share the torque.
Torque vectoring differentials on front or rear wheel drive vehicles are less complex than all-wheel drive differentials. The two wheel differential only shares torque between two wheels.
A front-wheel drive differential must take into account several factors. It must monitor rotational and steering angle of the wheels. As these factors vary during driving, different forces are exerted on the wheels. The differential monitor these forces, and adjusts torque accordingly. Many front-wheel drive differentials can increase or decrease torque transmitted to a certain wheel by changing the ratio between the two wheels. This ability improves a vehicle's capability to maintain traction in poor weather conditions. When one wheel begins to slip, the differential can reduce the torque to that wheel, effectively braking the wheel. The differential also increases torque to the opposite wheel, helping balance the power output and keep the vehicle stable. A rear-wheel drive torque vectoring differential works the same way as a front-wheel drive differential, but doesn't monitor the steering angle.
Most mechanical torque vectoring differentials are on all-wheel drive vehicles. A first torque vectoring differential varies torque between the front and rear wheels. This means that under normal driving conditions, the front wheels receive a set percentage of the engine torque, and the rear wheels receive the rest. If needed, the differential can transfer more torque between the front and rear wheels to improve vehicle performance.
For example, a vehicle might have a standard torque distribution of 90% to the front wheels and 10% to the rear. Under harsh conditions, the differential changes the distribution to 50/50. This new distribution spreads the torque more evenly between all four wheels. Having more even torque distribution increases the vehicle's traction.
There are more advanced torque vectoring differentials as well. These differentials build on basic torque transfer between front and rear wheels. They add the capability to share torque between a pair of front wheels or a pair of rear wheels.
The differential monitors each wheel independently, and distributes available torque to match current conditions. One known mechanism first transfers power between front and rear pairs and subsequently shares the amount of torque transmitted to each rear wheel by a second differential in series. The front wheels, however, do not receive different amounts of torque. Another known torque vectoring system adds a third mechanical differential to share torque provided to the front pair of wheels.
Another known system supports 4 electric motors coupled by gearboxes and axles to individual wheels. Negative torque is produced electrically rather than applying brakes as mechanical systems do.
As is known, Mercedes Benz has provided a purpose built electric vehicle with four synchronous independent electric motors. The engines make a total of 740 (750 PS) and 1,000 Nm (737.5 lb-ft), which is split equally among the four wheels in normal driving conditions. Because all four motors are electrically-powered independently of one another translates into potentially high speed wheel control.
The conventional Mercedes approach are still mechanically linking each motor to its wheel by a reduction gearbox and axle. A much more economical Tesla utilizes a single 3 phase AC induction motor and has a conventional mechanical power train. A conventional mechanical power train provides three differentials and reduction gearboxes. A conventional power train must have the same reduction ratio from engine to the front axis as well as to the rear axis to enable all wheel drive.
It is known that torque vectoring is particularly suited to electric vehicles. Lotus has been evaluating and developing new systems and approaches. When a driver turns the steering wheel, they expect the vehicle to change direction (yaw). The vehicle does not, however, respond immediately because tires take time to build up lateral forces, and the actual vehicle response may not be exactly what is required, or expected.
Particularly at high vehicle speed, after an initial delay period (a fraction of a second) the vehicle yaw rate can overshoot and oscillate before settling on a steady value. At very high speeds, or if the vehicle's suspension is poorly tuned or the operator poorly skilled, the oscillations can increase and the vehicle can go out of control. Even at lower speeds, the oscillations can make the vehicle feel less stable and the driver may need to make multiple steering adjustments to successfully follow the intended path.
Conventional vehicle suspension is tuned through bump steer, static settings, etc, to minimize the oscillations and to give a stable response at all vehicle speeds and loading conditions, but any increase in stability is at the expense of vehicle agility and the vehicle response can become disappointing.
It is known that when a vehicle has independent control over the drive and braking torques to each wheel (for instance, electric hub motors), there is an opportunity to improve the vehicle yaw response.
One approach has been by increasing the drive torque to a pair of tandem wheels (e.g. port), and creating an effective braking torque at the opposite pair of tandem wheels (e.g. starboard). These drive torques are in addition (or subtracted from) to the normal drive torques required to control vehicle speed. In other words, turning or yaw occurs when one side of a car is traveling faster than the other side.
Maximum Yaw Turning Moment (Torque)
Independent of the steered angle of the wheels, a yaw moment is generated when the resultant vector of the tire forces is perpendicular to a line through the center of gravity. The resultant force is the vector sum of lateral force and driving/braking force. The maximum yaw moment (if required) is obtained when the resultant of the tire forces is perpendicular to a line from the center of the tire to the vehicle center of gravity.
There are two main advantages in using these resultant forces to control vehicle yaw (as opposed to purely tire lateral forces): a. The resultant force can act at a greater lever arm, increasing the maximum moment available. b. Yaw rate can be controlled without requiring any steering.
If the forces are correctly controlled, the vehicle can be made to respond more quickly to a steering input and instability can be reduced.
To do this, the control of the wheel torques needs to consider: a. Increasing torque on the one side must be balanced by a reduction on the other side to avoid unnecessary acceleration. b. Vertical load on each wheel—particularly as the vehicle corners, c. the vertical load on the inner wheels reduce and drive/braking torque may cause wheel spin or wheel lock-up. c. The addition of drive or braking torques at the rear may result in loss of rear grip—leading to loss of control. d. Any response must be safe and predictable.
Therefore, simply distributing the torque based on steering wheel angle would achieve more yaw response (for the same steering input), but it may not create any improvement in stability. It could even make the vehicle behavior less predictable.
One known approach is yaw rate feedback. For any steer angle and forward velocity, an ideal yaw rate can be calculated by assuming no tire slip, and using the wheel geometry to approximate the turn radius. The measured yaw rate is then used as feedback, giving a yaw error. A differential term (yaw acceleration) is included for damping. The output is used to control the distribution of drive torque; i.e. for a left turn, an additional torque is applied to the right, with an equal braking torque applied to the left. These torques are in addition to the ‘normal’ drive torque that maintains the vehicle forward velocity.
A limitation to conventional feedback control is that the system relies on measured yaw rate as an input signal. This measured response data will also include ‘noise’ (high frequency waves created by road inputs and general vibration). In order to use the signal, the signal must be filtered. This unfortunately creates a time delay in the signal, and the feedback becomes too late creating overshoot and oscillations in the response.
Electric Motor or Traction Drive Controls
Transmitting positive or negative values in Newton meters.
It is known that Direct Torque Control provides used in variable frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor.
See patents by Depenbrock, takahashi and Noguchi direct self control and direct torque control.
U.S. Pat. No. 4,678,248 discloses a method for controlling a rotating-field machine supplied from an inverter, the output voltage system of the inverter being variable with respect to amplitude, phase and frequency includes supplying amplitudes of stator flux components formed from measured stator current components and stator voltage components as actual value of a flux control loop, and changing the phase and frequency of the inverter output voltage system with a flux control as a function of a predetermined stator flux reference value by directly setting-in the switching state of the inverter and an apparatus for carrying out the method.
It is known that Vector motor control or field-oriented control provides control over three-phase AC electric motors by adjusting the output current of a VFD inverter in Voltage magnitude and Frequency. FOC is a control technique that is used in AC synchronous and induction motor applications that was originally developed for high-performance motor applications which can operate smoothly over the full speed range, can generate full torque at zero speed, and is capable of fast acceleration and deceleration but that is becoming increasingly attractive for lower performance applications as well due to FOC's motor size, cost and power consumption reduction superiority. Not only is FOC very common in induction motor control applications due to its traditional superiority in high-performance applications, but the expectation is that it will eventually nearly universally displace single-variable scalar volts-per-Hertz (V/f) control.
What is needed is an improved apparatus and method to enable dynamic wheel control for energy and torque budgeting for each wheel.